Case study/ Codecov

Created by: st0nebraker

Closes #5584 (closed), adds new Codecov case study

Changelog

• Adds new Codecov case study in accordance to new case study layout.

Test

1. Ensure prettier has standardized the proposed changes.
2. Nav to /case-studies/codecov-uses-sourcegraph-to-resolve-incidents-12-times-faster and ensure copy & spec match.